Vacuum tubes (also called electron tubes, vacuum bulbs, valves, or simply, tubes) are sealed glass tubes with metal pins sticking out of one end so that the tube can be plugged into a socket as part of an electronic circuit. Inside the tube are various elements—metal cylinders or boxes, coils, and wires to keep everything suspended inside the tube. There is no air (almost no air) inside a vacuum tube, which is a requirement for the tube to perform its electrical function. The vacuum tube was created in 1904, and they were actively used in electronics in the US until the 1960s, when tubes were gradually phased out and replaced by the next major innovation—transistors. Unlike many “new old stock” items manufactured more than 50 years ago, unused vacuum tubes are inherently preserved—the inner workings are as pristine as the day they were made since there is no air or moisture to cause oxidation (assuming that the seal between the glass tube and the external pins hasn’t failed). Therefore, if you find an unused tube, chances are excellent that it will work as-is in your vintage electronic device.
Vacuum tubes work by controlling a stream of electrons that are speeding from one element to the next through the empty space in the tube. Everything starts at the cathode, an element of the tube that is coated with a special material that loves to release electrons when it is heated. The simplest cathodes are just a coiled filament like you would find in an incandescent light bulb. The ends of the filament are connected to two pins at the tube base, and by passing the right amount of electric current through the filament, we can heat it red hot, giving the tube its characteristic red glow.
Once the filament is red hot, it starts releasing electrons. Now we need another tube element in order to do something useful. This new element is called the plate. In its simplest form, it’s just a flat piece of metal, though often it takes the shape of a cylinder that wraps around the long, thin wire of the cathode without touching it. This simple type of tube with two elements, cathode and plate, is called a diode.
Let’s talk about the function of the plate. You may have heard the phrase “opposite charges attract.” Most likely, you’ve experienced this phenomenon if you’ve tried to comb or brush your hair on a dry day. The action of rubbing the comb or brush against your hair strips electrons off the hair. Electrons are negatively charged, and the removal of electrons leaves your hair positively charged, causing it to stick to the now negatively charged comb or brush.
Now, what if we connect a battery between the cathode and the plate? This will change the electrical potential, or voltage, between these two tube elements. If we give the plate a positive potential as compared to the cathode, the negatively charged electrons coming off the cathode will be attracted to the plate and will fly through the space in the tube to the plate. The metal plate is a good conductor, so the electrons will continue to flow into the plate and through the connecting wire to the pin, where they will exit the tube. This flow of electrons from battery to cathode, through the empty space in the tube to the plate, and back out of the tube to the battery is an electric current.
Let’s turn the battery around so that the plate now has a negative potential as compared to the cathode. If the plate is negative, it will no longer attract the negatively charged electrons that are coming off the cathode, and no electric current will flow as in the previous configuration. This is the key feature of a diode: it will only allow an electric current to flow in one direction.
Because diodes only allow electric current to flow in one direction, they are often used to change the alternating current (AC) that comes out of an electrical outlet into direct current (DC) that is needed by many types of circuits found in consumer electronics. The current that comes out of an outlet changes direction many times per second, hence the name “alternating current.” One cycle of alternating current begins with no current flowing. The current begins flowing in one direction and increases until it reaches a maximum. Then, the current decreases until it stops for an instant, at which point it reverses direction and again increases to a maximum in the opposite direction. Once again, the current decreases until it reaches zero, at which point the cycle is complete.
You may have heard the term “sine wave” used to refer to this graph. The current that comes out of an outlet in the US goes through 60 of these cycles per second, and most countries use either 60 cycles per second or 50 cycles per second. When we connect a diode in series with the current coming out of the outlet, it only allows current to flow in one direction. The process of converting AC into DC is called rectification and another name for a diode is rectifier.
As you can see in the graph above, which shows two cycles of rectified AC, the diode only allows current to flow for one half of each cycle. This is called half-wave rectification. The current flows in only one direction, but it changes in magnitude, and sometimes there isn’t any current flowing. This type of current flow is also called “pulsed DC.” We need a few more components in our circuit to smooth out this current and make it truly constant, like the current that flows out of a battery, but that can be the subject of a future blog post.
As it turned out, diode tubes also found an important use in radio. Radio signals are transmitted as electromagnetic waves that set up a very weak alternating current in an antenna. Before the information encoded in this alternating current can be heard, we must pass it through a diode, once again changing the AC into pulses of current that flow in only one direction. This process is called detection, and tubes used to do this are sometimes referred to as detectors. Before the advent of the diode, early radio systems used clunky mechanical methods to detect incoming signals. This worked okay for morse code, a series of short and long pulses that are used to encode the letters of a message. However, because they have no moving parts, vacuum tube diodes can react to incoming radio signals much more quickly.
Because of their fast response time to incoming signals, vacuum tube diodes enabled the reception of voice and music. Radio transmissions were no longer limited to morse code, which made radio more accessible to the general public. Radios require direct current to operate, and vacuum tube diodes also enabled rectification of AC into DC, so that radios could be plugged into a wall outlet. This eliminated the need to buy expensive batteries and allowed radio owners to play their sets as much as they liked, without worrying that the batteries would die.
In my next post, I’ll talk about how the addition of a third tube element, called a grid, created a tube called a triode. It was the triode that kicked the field of electronics into high gear—radio sales skyrocketed and millions of homes started listening to a growing number of stations. Even more impressive, triodes paved the way for many of the modern electronics we now take for granted, such as televisions and computers.